In the past, a control system of an internal combustion engine which is provided with an air-fuel ratio sensor in an exhaust passage of an internal combustion engine and controls the amount of fuel which is fed to the internal combustion engine based on an output of the air-fuel ratio sensor, has been widely known. As such a control system, one which is provided with an air-fuel ratio sensor at an upstream side of an exhaust purification catalyst which is provided in an engine exhaust passage and with an oxygen sensor at a downstream side, has been proposed (for example, PTL 1).
In particular, in the control system described in PTL 1, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is controlled so that the oxygen storage amount of the exhaust purification catalyst becomes a certain target value. Specifically, when the oxygen storage amount of the exhaust purification catalyst is larger than a target value, feedback control is performed so that the output air-fuel ratio of the upstream side air-fuel ratio sensor becomes an air-fuel ratio which is richer than the stoichiometric air-fuel ratio (below, referred to as “rich air-fuel ratio”). Conversely, when the oxygen storage amount of the exhaust purification catalyst is smaller than the target value, feedback control is performed so that the output air-fuel ratio of the upstream side air-fuel ratio sensor becomes an air-fuel ratio which is leaner than the stoichiometric air-fuel ratio (below, referred to as “lean air-fuel ratio”).
In addition, in the control system described in PTL 1, when the output of the downstream side oxygen sensor indicates a rich air-fuel ratio or lean air-fuel ratio for a given time period, the output of the upstream side air-fuel ratio sensor is corrected. Accordingly, it is considered that even if there is error in the output of the upstream side air-fuel ratio sensor, the oxygen storage amount of the exhaust purification catalyst can be made to match with the target value.